Colony stimulating factor promotes the proliferation and differentiation of bone marrow cells into macrophages and granulocytes. It also may act as a biological-response modifier. We have shown that purified CSF stimulates murine macrophages to inhibit tumor cell proliferation. In this project the anti-tumor potential of CSF will be explored in both in vitro and in vivo studies. Experiments will be designed to investigate the regulation of CSF-induced anti-tumor activity in macrophages. The role of "second signals" for activation such as lipopolysaccharide as well as the role of interferon and prostaglandins in the activation process will be investigated. In addition, the properties of CSF-stimulated macrophages will be explored. The mechanisms of macrophage-tumor inhibition will be studied by investigating supernatant activity, the role of O2 dependent cytolytic mechanisms, the effect of various metabolic inhibitors, and the role of arginase and thymidine. In vivo experiments will be carried out to determine the potential of CSF as an anti-tumor agent. Mice will be injected with CSF and subsequently or simultaneously challenged with tumor cells. The growth of tumors will be monitored by mortality and by direct measurement of tumor mass. The methods used will include in vitro assays of macrophage-tumor inhibition which we have developed in this laboratory. These include the inhibition of tritiated thymidine incorportation by tumor cells and the inhibition of tumor cell proliferation in a colony growth assay. The in vivo tumor model utilizing P815 tumor cell and DBA/2 mice was previously used by others and has been established in our laboratory. CSF has distinct advantages as an anti-tumor agent. Its action is presumably by specific stimulation of macrophages. Consequently, use of this compound will avoid many of the toxic effects which may accompany drugs or other biological modulators. As CSF can be purified to homogeneity, the accompanying side effects due to impurities should be minimal. In addition, since CSF can be prepared from supernatants of cell lines grown in vitro, supplies of the material should be adequate. Thus, CSF may prove to be a prototype of new, endogenously produced anti-tumor substances, which will offer significant advantages over conventinal anti-tumor therapy.